1. Field of the Invention
The present invention relates to pressure sensors. More particularly, the present invention relates to soft pressure sensors that are sensitive to mechanical stress.
2. Description of the Related Art
Pressure sensors have a wide range of applications from industrial to personal usage. A variety of pressure sensing technologies such as potentiometric, inductive, capacitive, piezoelectric, piezoresistive, optical, flow, and strain gauge pressure sensing have been developed for various applications. However, due to their large size, high rigidity and inflexibility, most of these conventional pressure sensing devices are not wearable by a user, and thus, not suitable to be used for personal usage. Soft and flexible pressure sensors can be used on the surface of three-dimensional shape for mapping or measuring pressures, for example, in athletic sportswears, undergarments, or patients' casts.
Several types of pressure sensors with textile-like properties have been developed. For instance, U.S. Pat. No. 7,348,506 disclosed a linear pressure sensor which comprises both electrically conductive fibers and electrically insulating fibers. If no pressure is applied, the conductive fibers are separated by the insulating fibers. While with pressure exerted on, electricity is conducted between the conductive fibers.
U.S. Pat. No. 6,543,299 disclosed a two dimensional array consisting of a lattice of individual force or pressure sensor elements comprising intersecting pairs of elongated, flexible strands or threads. Each of the strands or threads consists of a central electrically conductive wire core having a low resistivity, covered with a piezoresistive material having a relatively higher electrical resistivity. The strands or threads are arranged into two parallel planar sets, one set forming parallel spaced apart rows and the other set forming parallel spaced apart columns angled with respect to the rows. Rows and columns of piezoresistive threads are retained in physical contact with one another at cross-over intersection points forming a lattice of piezoresistive junctions comprising individual force sensing elements, either by being bonded between a pair of thin, flexible, upper and lower laminating sheets, or by being interwoven to form a fabric mesh. The electrical resistance at contacting intersections decreases in a predetermined way with applied normal force, thus enabling quantitative measurement of the force by measuring the electrical resistance of the node.
U.S. Pat. No. 7,365,031 disclosed a pressure sensitive textile including two crossed electrical conductors in a woven, knitted, non-woven or plaited fabric. The conductors being normally biased apart at the crossover point with an air gap between them whereby the application of pressure normal to the plane of the fabric causes the conductors to make contact.
U.S. Pat. No. 6,826,968 disclosed a bidimensional capacitive pressure sensor includes a plurality of capacitors formed by two mutually orthogonal sets of plates parallel or substantially parallel to each other separated, at least in correspondence of the crossing areas between electrodes belonging to one and the other set, by a layer of an elastically compressible dielectric material, having an array or matrix of column plates and row plates separated, at least at crossings, by elastically compressible dielectric. The system for biasing and reading capacitances comprises circuits for selecting a column and a row plate and sequential control logic circuitry of the column and row selection circuits for generating read values of the pressure each relative to a single pixels represented by capacitor realized in the superposition or crossover area of a selected column plate with a row plate.
In E.P. Pat. No. 1,605,240, a sandwich-structured sensor with electrodes each side of a central compressible layer is disclosed. The compressible layer has a reversible effect and is applied onto a flexible base electrode layer and is covered by an insulating layer. The sensor is completed by a pattern of flexible electrodes of selected shapes applied to the insulating layer and with each electrode with a separate connection to a processor. The electrodes can be conducting fabric, i.e. with conducting thread, or can be printed onto the support layers. A capacitive pressure sensor with similar configuration is also discussed in PCT Pat. Appl. No. WO2005/121729.
The above-mentioned sensors have numerous shortcomings such as non-adjustable pressure ranges, limited service lifetime, limited sensitivity, and complexity.
In view of the above conventional pressure sensing devices, there are still needs for a pressure sensing device that is simple, compact, durable, reliable, cost-effective, lightweight and flexible with excellent sensitivity and adjustable pressure sensing ranges.